Field of the Invention
This invention relates generally to gas turbine engine combustors and, more particularly, to heat shields on a combustor dome in the gas turbine engine combustor.
Description of Related Art
Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) generated as a result of gas turbine engine operation. In particular, nitrogen oxide is formed within a gas turbine engine as a result of high combustor flame temperatures. Making modifications to a gas turbine engine in an effort to reduce nitrous oxide emissions often has an adverse effect on operating acoustic levels of the associated gas turbine engine.
Destructive or undesirable acoustic pressure oscillations or pressure pulses may be generated in combustors of gas turbine engines as a consequence of normal operating conditions depending on fuel-air stoichiometry, total mass flow, and other operating conditions. The current trend in gas turbine combustor design towards low NOx emissions required to meet federal and local air pollution standards has resulted in the use of lean premixed combustion systems in which fuel and air are mixed homogeneously upstream of the flame reaction region. The fuel-air ratio or the equivalence ratio at which these combustion systems operate are much “leaner” compared to more conventional combustors in order to maintain low flame temperatures which, in turn, limits production of unwanted gaseous NOx emissions to acceptable levels.
This method often uses water or steam injection for achieving low emissions, but the combustion instability associated with operation with water or steam injection and at low equivalence ratio also tends to create unacceptably high dynamic pressure oscillations in the combustor that can result in hardware damage and other operational problems. Pressure pulses can have adverse effects on an engine, including mechanical and thermal fatigue to combustor hardware. The problem of pressure pulses has been found to be of even greater concern in low emissions combustors since a much higher percentage of air is introduced to the fuel-air mixers in such designs.
Dry-low-emissions (DLE) combustors are prone to combustion acoustics and typically include design features and/or control logic to reduce the severity of combustion acoustics. These include acoustic damper, multiple fuel systems, and supplemental fuel circuits. Multiple fuel systems allow for flame temperature variation within the combustion chamber. The LM2500 DLE and LM6000 DLE incorporate three rings of premixers that are independently fueled. This allows for the outer, middle, and inner premixers to have different flame temperatures.
Supplemental fuel circuits have been used to inject a relatively small portion of the fuel into the combustor at different locations from the primary injection locations. This out-of-phase fluctuation in heat release serves to reduce the amplitude of the pressure fluctuations. In some implementations, the supplemental fuel also introduces temperature variation within the combustion chamber.
In at least some of the General Electric LM2500 DLE and LM6000 DLE combustors, supplemental fuel is injected from every other premixer. The fuel flow to premixers without supplemental fuel is generally lower than those with the supplemental fuel.
At least some known gas turbine combustors include a plurality of mixers which mix high velocity air with liquid fuels, such as diesel fuel, or gaseous fuels, such as natural gas, to enhance flame stabilization and mixing. At least some known mixers include a single fuel injector located at a center of a swirler for swirling the incoming air. Both the fuel injector and mixer are located on a combustor dome. A typical dome includes a dome plate supporting heat shields. The combustor includes a mixer assembly and heat shields that facilitates protecting the dome. The heat shields are cooled by air impinging on the dome to facilitate maintaining operating temperature of the heat shields within predetermined limits.
During operation, the expansion of the fuel-air mixture flow discharged from a pilot mixer may generate toroidal vortices around the heat shield. Unburned fuel may be convected into these unsteady vortices. After mixing with combustion gases, the fuel-air mixture ignites, and an ensuing heat release can be very sudden. In many known combustors, hot gases surrounding heat shields facilitate stabilizing flames created from the ignition. However, the pressure impulse created by the rapid heat release can influence the formation of subsequent vortices. Subsequent vortices can lead to pressure oscillations within combustor that exceed desirable or acceptable limits.
It is highly desirable to have an effective means for eliminating or reducing these high levels of noise or acoustics in a gas turbine engine combustor, particularly, one that has a short length and is designed for low NOx (nitrous oxides), CO, and unburnt hydrocarbon emissions. It is also highly desirable for this means to be simple to employ or add to already existing engines and to tune it for specific engines and installations. Conical outer and inner heat shields on combustor domes are disclosed in U.S. Pat. No. 8,596,071 issued to Mark Anthony Mueller, et al., Dec. 3, 2013. U.S. Pat. No. 8,596,071 is assigned to the present assignee, General Electric Company, and incorporated herein by reference.
Combustion instability is a challenging problem in DLE combustors in which the fuel is burned in a lean premixed flame. Combustion instability in some cases could create large acoustic pressures that can drive structural vibrations, high heat fluxes to combustor walls, flame flashback (by longitudinal mode) and flame blow-off (by tangential or radial modes). In some extreme cases, the outcome is engine hardware failure. One of the most effective ways to eliminate combustion instability is to anchor the lean-premixed flame on a well-designed flame-holder such that the space lag is outside of instability domain. For this reason, it has been demonstrated that combustor dome heat shield design and shape (as a flame holder) has a paramount effect on driving suppression of combustion acoustics.